1. Technical Field
The present invention relates to platinum-copper alloy electrocatalysts and to acid-electrolyte fuel cell electrodes using the same.
2. Description of the Prior Art
The fuel cell is an electrochemical device for direct conversion of a fuel, such as hydrogen gas or hydrocarbons, and an oxidizing agent, such as oxygen gas, into a low-voltage direct current. It generally comprises a fuel electrode (anode), an oxidizer electrode (cathode), an electrolyte placed between the two electrodes, and a means to separately introduce fuel and oxidizer streams to the anode and to the cathode, respectively. Electrocatalysts are used in both the anode and cathode. In operation, the fuel, which is supplied to the anode and brought into contact with the electrocatalyst, is oxidized in the presence of the electrolyte, thereby liberating electrons. The oxidizing agent, on the other hand, is fed to the cathode, where it is reduced on the surface of the electrocatalyst in the presence of the electrolyte, thereby consuming the electrons transferred from the anode via an external circuit and generating electric power. Thus the electrocatalysts used in the anode and cathode play a very important role in a fuel cell; the output efficiency and service life of a fuel cell depend greatly upon the activity of the electrocatalysts used.
It is known that, of the Group-8, Group-9 and Group-10 metals of the Periodic Table (according to the nomenclature recommended by IUPAC in November, 1983), the "platinum group metals" (Pt, Pd, Rh, Ru, Ir and Os) can be advantageously used, either alone or in combination, as the electrocatalyst. It is common practice that such a platinum group metal, or a combination thereof, is supported on a conductive carrier material, such as conductive carbon black, in a well dispersed form, and the catalyst thus obtained is fixed to a support member, such as a metal screen (made of, for example, nickel or tantalum) or waterproof graphite paper, thus making up an electrode.
It is also known that in oxygen/hydrogen feed phosphoric acid fuel cells, in particular, the activation polarization of oxygen reduction at the cathode is far greater in magnitude than that of hydrogen oxidation at the anode. When a supported platinum-group metal mentioned above is used as a cathode, the activity of the catalyst tends to gradually decline, leading to a lowering in the output and overall operation efficiency of the cell. Presumably, the crystallites of the supported platinum-group metal on the catalyst tend to be dissolved out into the electrolyte, or to grow into large particles as a result of sintering during cell operation by the action of oxygen and electrolyte held at about 200.degree. C., thereby decreasing the specific surface area of the active metal and resulting in the decline in the catalyst activity.
Assiduous studies have recently been conducted with respect to the development of the phosphoric acid fuel cell, but it is generally accepted that, in order for this type of fuel cell to come into widespread use as a power generating system, it must be able to continue operation over a period of more than 40,000 hours at a power output above a certain level. This requirement cannot be met so long as an electrode comprising a platinum-metal catalyst as mentioned above is used.
Thus there has been a demand for new platinum-based catalysts which will maintain higher activity over longer periods. To this end, it would be important to enhance the specific activity of the supported metal (activity per unit surface area of supported metal), and also to enhance the maintainability of its specific surface area (surface area per unit weight of supported metal).
Many studies have so far been made on supported metal catalysts with higher specific activity. These include alloys of a platinum group metal with various other metals, primarily Group 2 to 6 base metals such as vanadium, tungsten, aluminum, titanium, silicon, cerium, strontium and chromium (U.S. Pat. No. 4,186,110, No. 4,202,934 and No. 4,316,944); ternary alloys prepared by adding cobalt to platinum-vanadium or platinum-chromium alloys (U.S. Pat. No. 4,447,506); alloys of a platinum group metal with gallium, or superlattice alloys between a platinum group metal and iron (Japanese Patent Application Laid-open No. 7941 and No. 156551 (1985)); and ternary alloys prepared by adding cobalt and nickel to platinum (Japanese Patent Application Laid-open No. 8851 (1986)). These alloys show enhanced specific activity, but little improvement can be achieved in the maintainability of specific surface area.
As stated above, the decrease in the specific surface area of supported metal is caused by sintering of the metal particles or their dissolution into the electrolyte. Several techniques have been proposed to prevent this difficulty. These include a method in which the supported metal is treated with a carbon-containing substance, such as carbon monoxide and hydrocarbon gases, followed by calcination in an inert gas atmosphere, thereby forming carbon film over the supported metal particles and the surrounding areas (U.S. Pat. No. 4,137,372); a process in which the active metal is supported on a carrier made of a non-conductive material, and the supported metal thus prepared is mixed with powder of conductive carbon to make up an electrocatalyst (a process based on the concept that the sintering of supported metal particles is caused by electrochemical corrosion due to conductivity of the carrier) (Japanese Patent Application Laid-open No. 212961 (1985)); and a method of making the supported active metal less wettable to the corrosive electrolyte by increasing the amount of waterproof polymer added to the catalyst layer in order to enhance the gas diffusivity of electrode. None of these methods, however, is satisfactory in enhancing the specific activity of the supported metal, although some improvement is observed in the maintainability of its specific surface area.
As described above, conventional fuel cell electrocatalysts fail to maintain high activity over long periods because of the low maintainability of specific activity and/or specific surface area of supported metal, and therefore fuel cells using such an electrocatalyst are not satisfactory in output power and service life.